Gamma-aminobutyric acid

Gamma-aminobutyric acid (usually abbreviated to GABA) is an inhibitory neurotransmitter found in the nervous systems of widely-divergent species. It is the chief inhibitory neurotransmitter in the central nervous system and also in the retina. GABA is an amino acid, but is not found in proteins. Although some GABA can be found in pancreatic islet cells and kidney, there are no significant amounts of GABA in mammalian tissues other than the tissues of the nervous system.

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Function

In vertebrates, GABA acts at inhibitory synapses in the brain. GABA acts by binding to specific transmembrane receptors in the plasma membrane of both pre- and postsynaptic neurons. This binding causes the opening of ion channels to allow the flow of either negatively-charged chloride ions into the cell or positively-charged potassium ions out of the cell. This action results in a negative change in the transmembrane potential, usually causing hyperpolarization. Three general classes of GABA receptor are known: GABA_A and GABA_C ionotropic receptors, which are ion channels themselves, and GABA_B metabotropic receptors, which are G protein-coupled receptors that open ion channels via intermediaries (G proteins).

Neurons that produce GABA as their output are called GABAergic neurons, and have chiefly inhibitory action at receptors in the adult vertebrate. Medium Spiny Cells are a typical example of inhibitory CNS GABAergic cells. GABA exhibits excitatory actions in insects, mediating muscle activation at synapses between nerves and muscle cells, and also the stimulation of certain glands. In hippocampus and neocortex of the mammalian brain, GABA has primarily excitatory effects early in development, and is in fact the major excitatory neurotransmitter in many regions of the brain prior to the maturation of glutamate synapses - See developing cortex. Whether GABA is excitatory or inhibitory depends on the direction (into or out of the cell) and magnitude of the ionic currents controlled by the GABA_A receptor. When net positive ionic current is directed into the cell, GABA is excitatory, when the net positive current is directed out of the cell, GABA is inhibitory. A developmental switch in the molecular machinery controlling the polarity of this current is responsible for the changes in the functional role of GABA between the neonatal and adult stages.

In spastic cerebral palsy in humans, GABA cannot be absorbed properly by the damaged nerve rootlets leading to certain muscles; this leads to hypertonia in those muscles.

Structure and conformation

GABA is found mostly as a zwitterion, that is, with the carboxyl group deprotonated and the amino group protonated. Its conformation depends on its environment. In the gas phase, a highly folded conformation is strongly favored due to the electrostatic attraction between the two functional groups. The stabilization is about 50 kcal/mol, according to quantum chemistry calculations. In the solid state, a more extended conformation is found, with a trans conformation at the amino end and a gauche conformation at the carboxyl end. This is due to the packing interactions with the neighboring molecules. In solution, five different conformations, some folded and some extended are found as a result of solvation effects. The conformational flexibility of GABA is important for its biological function, as it has been found to bind to different receptors with different conformations. Many GABA analogues with pharmaceutical applications have more rigid structures in order to control the binding better.^[1][2]

History

Gamma-aminobutyric acid was first synthesized in 1883, and was first known only as a plant and microbe metabolic product. In 1950, however, GABA was discovered to be an integral part of the mammalian central nervous system.^[3]

Synthesis

Organisms synthesize GABA from glutamate using the enzyme L-glutamic acid decarboxylase and pyridoxal phosphate as a cofactor. It is worth noting that this process converts the principal excitatory neurotransmitter (glutamate) into the principal inhibitory one (GABA).

Pharmacology

Drugs that act as agonists of GABA receptors (known as GABA analogues or GABAnergic drugs) or increase the available amount of GABA typically have relaxing, anti-anxiety and anti-convulsive effects. Many of the substances below are known to cause anterograde amnesia and retrograde amnesia.

GABA has been purported to increase the amount of the Human Growth Hormone. The results of those studies have been seldom replicated, and have recently been in question since it is unknown whether GABA can pass the blood-brain barrier.

Drugs that affect GABA receptors:

• alcohol (ethanol)^[4][5][6]

• avermectins—doramectin, selamectin, ivermectin
• barbiturates
• bicucullines - GABA antagonist
• benzodiazepines
• baclofen
• baicalin and baicalein from skullcap scutellaria lateriflora
• carbamazepines^[7]
• cyclopyrrolone derivatives such as zopiclone
• fluoroquinolones
• gabazine (SR-95531)
• gamma-Hydroxybutyric acid (GHB)^[8]
• gamma-amino-beta-hydroxybutyric acid
• imidazopyridine derivatives such as zolpidem
• kavalactones^[9]
• muscimol
• phenytoin
• picamilon
• picrotoxin
• progabide
• propofol
• phenibut
• pyrazolopyrimidine derivatives such as zaleplon
• thujone—GABA antagonist

Drugs that affect GABA in other ways:

• tiagabine—potentiates by inhibiting uptake into neurons and glia
• vigabatrin—potentiates by inhibiting GABA-T, preventing GABA breakdown
• valproate—potentiates by inhibiting GABA-T
• tetanospasmin—primary toxin of tetanus bacteria, blocks release of GABA
• hyperforin—inhibits the reuptake of GABA

See also

• Spastic diplegia

References